Heavy Oil in Deepwater — Issues and Observations
Barry J. Katz
ETC, Chevron, Houston, TX
Over the past two decades exploration has progressed into deepwater, where the current resource-base exceeds 100 BBOE. An examination of this resource reveals that about 60% of it is oil, largely associated with Atlantic basin margins. In areas such as offshore Mexico, Brazil, and Angola a significant percentage of oil is heavy. Such oils are challenged because deepwater economics requires both high flow rates and ultimate recoveries, while heavy oils tend to display high viscosities, reducing flow rates and recoveries. Many of the approaches used onshore and in shallow water to enhance flow rates of heavy oils are of limited value in deepwater. They do, however, provide guidance as how to maximize value. Studies reveal the need for continuous data collection and refinement of the reservoir model as well as the need to integrate across disciplines and the value chain to maximize return.
Once discovered the focus is on maximizing value. During the exploration phase the focus is a better risking of the probability of encountering heavy oil. This is accomplished through a clearer understanding of the controls on crude oil quality. Although there are a number of factors that may reduce oil quality biodegradation appears to be the dominant cause in deepwater, with the exception of offshore Mexico where source rock attributes play a greater role. Biodegradation is controlled by the charge and thermal histories and the geometry of the oil-water contact. In deepwater thermal histories tend to favor biodegradation, while the youthfulness of the reservoirs and charging histories tends to favor preservation. Mixing of oils, such as is the case offshore Brazil and Nigeria, also appears to be an important factor, often improving or maintaining quality, through the introduction of light oils into biodegraded oils.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery